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Roadway trees planted as barriers reduce traffic noise and particle pollution in cities. However, tree roots may alter the soil's moisture content, resulting in uneven soil subsidence and maintenance issues for neighbouring structures. This study determined the safe distance between trees and pavement by investigating the effects of transpiration on pore water pressure (PWP) and pavement subsidence under seasonal variations. The root water uptake was simplified in a finite element model using multiple hydraulic head boundaries and validated using field observations. A hypoplastic model was used to model non-linear behaviour and plastic strain accumulation in unsaturated soil. Evergreen trees can reduce PWP by 72% during the dry season and 84% during the wet season, compared to bare soil. The subsidence did not decrease linearly with the distance away from trees under a rigid and impermeable pavement structure. The maximum bending moment was influenced by the distance between trees and pavement with the highest value occurring when trees were located near the pavement (up to five times the bending moments on pavement without trees). Our findings suggest that the pavement is at risk of experiencing structural failure if trees are located within 0.4 times their height away from the pavement.

Seasonal variation of soil surface temperature, such as freezing and thawing, can induce increases in the water permeability in clay by mobilizing clay pore structure. This kind of weather-induced change in clay behavior may worsen the water-sealing performance during the construction and after closure operation of engineered structures such as soil barriers for tailings or man-made slopes. There is limited knowledge towards the influence of freezing–thawing cycles on clay microstructure and saturated permeability (Ksat). This study investigated the saturated permeability of clay under freezing–thawing cycles and explored the uses of biochar as eco-friendly amendment to manipulate the permeability of compacted clay. Clay specimens were compacted with different initial water contents (30%, 34%, and 38%). The biochar application rates of 0%, 2%, 4%, 8% (by dry weight) were applied to measure their effects on the permeability of clay specimens. Saturated permeability was measured by the falling head tests. Any variation of biochar amended clay microstructure after freezing–thawing cycles was captured by the scanning electron microscope. The Ksat was reduced by about one order of magnitude when the biochar application rate was larger than 4%. This may be attributed to the increase in the filling of the biochar particles in the clay intra-aggregate pores upon the transport of liquid water during the repeated freezing–thawing processes. The biochar may thus be recommended to minimize the Ksat of geo-environmental structures in cold regions when sufficiently large application rate was used to facilitate the pore-filling process.

Hydrochar is a biomass-derived carbon-rich material produced by the hydrothermal carbonization process which requires less energy than the pyrolysis production of biochar. The effectiveness of using hydrochar to amend soil properties, especially hydraulic conductivity, and the underlying mechanism that hydrochar follow remain unknown. This study measured the effects of grass feedstock and grass-derived hydrochar produced at two temperatures (180°C and 240°C) on the pore size distributions (PSDs) and saturated hydraulic conductivity (ks) of compacted silty-clay sand. Hydrochar affected the ks through predominantly the change of macropores of amended soil. Specifically, the addition of 180°C hydrochar [with a 60% specific gravity (GHs) of the soil] at the mass proportion (fH) of 2.5% evolved the PSD from unimodal to trimodal, creating a more open soil structure and increasing the ks by more than half an order of magnitude. When fH exceeded the threshold of 2.5%, the improvement of ks decreased in effectiveness following the compression of macropores. The 240°C hydrochar that has a larger GHs (than the 180°C case) has a high threshold of 5% and introduced a great increase in ks. Test results highlight the importance of avoiding adding excessive hydrochar to prevent the reduction of the effectiveness of drainage improvement.

Quantifying evolutions of the biomechanical properties and mechanical root reinforcement to soil with the duration of root decomposition is important to land management strategy and to soil stabilisation purposes. However, the variations of these properties of the roots of herbaceous species, especially following herbicide application in agriculture practices, have rarely been studied. This study aims to measure the effects of root decomposition due to herbicide on the root biomechanical properties and root reinforcement provided by two contrasting vetiver species (Chrysopogon nemoralis and Chrysopogon zizanioides). We applied herbicide (i.e., propanil) to four treatments of each species, considering four different durations of decomposition (7-, 28-, 56- and 112-days since herbicide application). The biomechanical properties were measured by uniaxial tensile tests, whereas the root reinforcement to poorly graded sand (SP) was quantified by direct shear tests. Root decomposition significantly reduced mean root tensile strength, secant modulus and breakage strain of C. nemoralis and C. zizaniodes roots after 112 days since the herbicide application. Significant negative power correlations between root diameter and root strength (or root secant modulus) (R2 = 0.39–0.86; p-value < 0.05) were identified. Root decomposition did not change the shape of these correlations, but they shifted downwards as roots decomposed. The root reinforcement also declined with the decomposition duration, in terms of root cohesion and maximum dilatancy within the study period. C. nemoralis displayed greater and quicker loss of both the root biomechanical properties and root reinforcement to soil than C. zizanioides. © 2022 John Wiley & Sons Ltd.

Plant root reinforcement in soil bioengineering has gained increasing interest as a means of sustainable and environmentally friendly soil reinforcement and stabilisation. While Chrysopogon zizanioides is widely distributed in tropical regions worldwide and has been advocated for use in slope stabilisation and soil erosion control, C. nemoralis is normally distributed in mountainous areas in Southeast Asian countries, and its potential to reinforce soil has rarely been explored in the soil bioengineering literature. With the importance of root properties in soil bioengineering, this study was carried out to provide a comprehensive dataset of root biomechanical properties, morphological traits, and root reinforcement of these two contrasting vetiver species. A series of experiments, including root observation with a rhizobox system, uniaxial tensile test, and direct shear test, was performed. Results showed that Young's modulus and diameter of C. nemoralis roots were almost 1.4- and 1.3-times greater than those of C. zizanioides roots (p < 0.05). By contrast, no significant difference between the two vetiver species was observed in terms of tensile strength, 'side' root area ratio (RARS), and root orientation (p > 0.05). The diameter–strength (R2 = 0.55–0.56, p < 0.05) and diameter–modulus relationships (R2 = 0.51–0.6, p < 0.05) of both species were consistent with negative power-law models. Conversely, their diameter–orientation relationship followed a linear model (R2 = 0.85–0.89; p > 0.05). The soil shear strength in terms of cohesion greatly increased in the presence of the roots of C. nemoralis (Δc = 4.9 kPa) and C. zizanioides (Δc = 4.4 kPa). Therefore, C. nemoralis could be an alternative to C. zizanioides in soil bioengineering applications. © 2021 John Wiley & Sons, Ltd.

Aim Root growth and decay may affect root reinforcement to soil erosion and stability. We measured the effects of growth and decay on the tensile strength of Cynodon dactylon roots considering different causes of mortality common to agricultural land conversion (i.e. burning and herbicide application). Method We applied three treatments to C. dactylon grass: (i) growth duration (60, 120 and 180 days), (ii) decay duration after burning (30, 60, 120, 180 and 360 days) and (iii) decay duration after herbicide application (15, 30 and 60 days). The diameter, tensile strength and cellulose and lignin contents of root samples (n = 303) in different treatments were measured. Results Tensile strength–diameter relations followed a negative power law regardless of treatment (R2 > 0.6). The increase in median tensile strength values due to grass growth was consistent with the increase in cellulose and lignin contents. Root decay by herbicide application caused significantly greater and faster reduction in tensile strength than burning treatment because of the faster reduction of cellulose and lignin contents. Conclusion Root decay due to different causes of plant mortality can increase susceptibility to erosion and slope instability during the conversion of agricultural land. Measures on slope safety and erosion are vital when using herbicides for weed clearance in farmlands due to the faster deterioration of root chemical composition and root tensile strength (compared with burning).

Removal of vegetation in close vicinity to transportation corridors would have an impact on slope stability due to the loss of hydro-mechanical root reinforcement. Fallowed slope wouldbe subjected to accelerated episodic movements and shallow failure, causing potential ultimate and serviceability limit state problems. This study investigates the effects of plant removal on slope hydrology and stability, through centrifuge and numerical modelling. This study uses artificial roots that are capable of simulating the effects of transpiration in-flight and have mechanical properties closely representative to real roots. Model slopes before and after plant removal (i.e. with and without considering transpiration effects, respectively) were subjected to identical intense rainfall in the centrifuge. Removing all vegetation on the slope lost all the beneficial effects of transpiration-induced suction that would otherwise be present without plant removal. Relying on mechanical reinforcements alone are insufficient to maintain slope stability. Preliminary parametric analyses reveal that to maintain slope stability, plant removal should not be conducted beyond two-third from the slope crest. Even removing only one-fourth of plants from the slope toe would significantly reduce factor-of-safety of slope and temporary support of the slope is needed.

Infiltration rate affects slope stability by determining the rate of water transport to potential failure planes. This note considers the influences of vegetation (grass and willow) establishment and root growth dynamics on infiltration rate, as related to establishing vegetation on bioengineered slopes. Soil columns of silty sand with and without vegetation were tested by constant-head infiltration tests at 2, 4, 6 and 8 weeks after planting. Infiltration rate increased linearly with plant age and below-ground traits including root biomass and root length density. Infiltration rate for willow-rooted soil was an order of magnitude higher than for fallow soil. The plant age effect was more prominent for willow, which grew faster and with thicker roots than the grass. Illustrative seepage analysis suggests that ignoring the plant age effects could underestimate wetting front advancement to greater depths during rainfall, and underestimate suction recovery at shallow depths during internal drainage.

Live poles have been used to stabilize shallow slopes. Most of the existing work has focused on pole reinforcement. The effects of pole transpiration on slope stability are unclear. This study aims to investigate the effects of pole transpiration on rainfall-induced slope hydrology through centrifuge model tests. A novel live-pole modeling technique using real branch cuttings was adopted to simulate the hydrological effects of the transpiration-induced suction in the centrifuge. Suction responses during rainfall were recorded and back-analyzed by seepage analysis. Higher suction was preserved after rainfall when pole transpiration induced higher suction before rainfall. This is because inducing higher suction would lower the soil water storage capacity and hydraulic conductivity. After a rainfall event with a return period of 20 years, up to 10 kPa of suction was preserved in the pole-supported slope to provide stabilization effects. (C) 2016 American Society of Civil Engineers.

Soil bioengineering using vegetation has been recognised as an environmentally friendly solution for shallow slope stabilization. Plant transpiration induces suction in the soil, but its effects on slope stability are often ignored. This study investigates the influences of transpiration-induced suction and mechanical reinforcement of different root geometries (i.e., tap and heart-shaped) to the slope stability subjected to an intense rainfall with an intensity of 70 mm/h (prototype scale; corresponding to a return period of 1000 years), via centrifuge modelling. New model roots that have scaled mechanical properties close to real roots were used to simulate transpiration-induced suction in the centrifuge. Transient seepage analyses were performed using SEEP/W to back-analyse the suction responses due to transpiration and rainfall. Subsequently, the back analysed suction was used to assess the factor of safety of the slopes using SLOPE/W. It is revealed that heart-shaped roots provided greater stabilization effects to a 60° clayey sand slope than tap-shaped roots. The heart-shaped roots induced higher suction, leading to 14% reduction of rainfall infiltration and 6% increase in shear strength. Although transpiration-induced suction in a 45° slope was reduced to zero after the rainfall, mechanical root reinforcement was found to be sufficient to maintain slope stability. © 2017, Canadian Science Publishing. All rights reserved.

Shallow landslides (i.e. 1-2-m depth) on both man-made and natural slopes are of major concern worldwide that has led to huge amount of socio-economical losses. The use of vegetation has been considered as an environmentally friendly means of stabilising slopes. Existing studies have focused on the use of plant roots with different geometries to mechanically stabilise soil slopes, but there are little data available on the contribution of transpiration-induced suction to slope stability. This study was designed to quantify both the hydrological and mechanical effects of root geometry on the stability of shallow slopes. Centrifuge tests were conducted to measure soil suction in slope models supported by newly developed artificial roots. These artificial roots exhibit three different representative geometries (i.e. tap, heart and plate) and could simulate the effects of transpiration. The measured suction was then back-analysed through a series of finite element seepage-stability analyses to determine the factor of safety (FOS). It is revealed that after a rainfall event with a return period of 1000 years, the slope supported by heart-shaped roots retained the highest suction within the root depth, and thus, this type of root provided the greatest stabilisation effects. The FOS of the slope supported by the heart-shaped roots, through both mechanical reinforcement and transpiration-induced suction, is 16 and 28 % higher than that supported by the tap- and plate-shaped roots, respectively.